1. Field of the Invention
The present invention relates to a method for producing a semiconductor device and a semiconductor device.
2. Description of the Related Art
The degree of integration of semiconductor integrated circuits, in particular, integrated circuits using MOS transistors has been increasing. With the increasing degree of integration, the size of MOS transistors used in integrated circuits has been decreased to nano-scale dimensions. Such a decrease in the size of MOS transistors causes difficulty in suppressing leak currents, which poses a problem in that it is hard to reduce the area occupied by the circuits because of the requirements of the secure retention of necessary currents. To address the problem, a surrounding gate transistor (hereafter referred to as an “SGT”) having a structure in which a source, a gate, and a drain are arranged vertically with respect to a substrate and a gate electrode surrounds a pillar-shaped semiconductor layer has been proposed (e.g., refer to Japanese Unexamined Patent Application Publication Nos. 2-71556, 2-188966, and 3-145761).
As the width of a silicon pillar decreases, it becomes more difficult to make an impurity be present in the silicon pillar because the density of silicon is 5×1022/cm3.
In known SGTs, it has been proposed that the channel concentration is set to be a low impurity concentration of 1017 cm−3 or less and the threshold voltage is determined by changing the work function of a gate material (e.g., refer to Japanese Unexamined Patent Application Publication No. 2004-356314).
It has been disclosed that, in planar MOS transistors, the sidewall of an LDD region is formed of a polycrystalline silicon having the same conductivity type as a low-concentration layer, surface carriers of the LDD region are induced by the difference in work function, and thus the impedance of the LDD region can be reduced compared with LDD MOS transistors with an oxide film sidewall (e.g., refer to Japanese Unexamined Patent Application Publication No. 11-297984). It has also been disclosed that the polycrystalline silicon sidewall is electrically insulated from a gate electrode. The drawings show that the polycrystalline silicon sidewall is insulated from a source and a drain by an interlayer insulating film.